Fluid power assist steering systems can include a pump for providing hydraulic pressure for operation of the assist steering system. Output pressure demands on the pump can vary with pump flow and system load restriction. However, the output pressure should be limited to a predetermined maximum output pressure in order to protect the fluid power assist steering system, the pump, and other system components from damage. The pump is typically provided with a relief valve to limit the output pressure.
The relief valve can include a spool having a poppet valve assembly. The poppet valve assembly includes a poppet spring that biases a poppet against a poppet seat. The poppet is moved away from the poppet seat if the predetermined maximum output pressure is exceeded, which causes a reduction in output pressure of the pump. The force applied by the poppet spring to the poppet determines the pressure at which the poppet valve assembly opens, thus establishing the predetermined maximum output pressure of the pump.
In prior art poppet valve assemblies, the force applied by the poppet spring is controlled by shims that are used to set the position of the poppet seat relative to an end of the spool. Varying the number of shims or using shims of different thickness adjusts the force applied to the poppet by the poppet spring. However, the use of shims requires disassembly and assembly of the relief valve every time an adjustment is made. Furthermore, the shims provide poor adjustment resolution of the predetermined maximum output pressure.